The development of resistance to chemotherapy drugs is a major limitation in the success of cancer treatment. Particularly problematic is the establishment of multidrug resistance (MDR); due to overexpression of the P-glycoprotein/MDR genes. The MDR genes encode a protein that is a member of a superfamily of membrane-bound transport molecules. This class of proteins contains a pair of adenosine triphosphate (ATP)-binding domains and a pair of hydrophobic domains. The ATP-dependent pump superfamily includes many prokaryotic permeases, the yeast mating factor transporter, the cystic fibrosis gene, and the HLA peptide transporter. MDR genes are capable of pumping a variety of hydrophobic drugs out of cells, including colchicine, vinblastine, and vincristine. We have designed a set of degenerate oligonucleotides to the ATP-binding domain of the MDR genes. These primers have been used in polymerase chain reactions to amplify sequences from Drosophila, Saccharomyces cerevisiae, and Escherichia coli. These products were cloned and sequenced. Each unique clone contained an open reading frame that had homology to the MDR genes, and no clone showed identity to any previously characterized genes. One clone from each species has been used to isolate genomic clones. The Drosophila sequence was mapped to chromosome 2, band 50; adjacent to the Hsc-5 gene. The yeast sequence maps to chromosome 7. The entire sequence has been determined and it contains a hydrophobic domain with 25% amino acid identity to human MDR1 and an ATP-binding domain 45% identical to human MDR1. The E. coli sequence contains a pair of ATP-binding domains 38-45% identical to human MDR1. Genetic experiments are in progress to try to determine the function of these new MDR-related genes. If these genes are capable of transporting drugs, they could prove to be useful models for better understanding the function of MDR genes, for designing effective chemotherapy agents, and for identifying inhibitors of MDR proteins.